Gas Balance and Fuel Retention in Fusion Devices

The evaluation of hydrogenic retention in present tokamaks is of a crucial importance to estimate the expected Tritium (T) vessel inventory in ITER, limited for safety considerations to 350g. In the frame of the European Task Force on Plasma Wall Interaction (EU TF PWI), efforts are underway to investigate the gas balance and fuel retention during discharges and compare with that from post-mortem tile analysis integrated over experimental campaigns. This paper summarizes the principal findings from coordinated studies on gas balance and fuel retention from a number of European tokamaks e.g. ASDEX-Upgrade (AUG), JET, TEXTOR and Tore Supra (TS). For most devices, the long-term retention fraction deduced from integrated particle balance is ~ 10-20% and is larger than the ~3-4% deduced from post mortem analysis of PFCs. However, from the database available from tokamaks with carbon as main PFCs, the important conclusion is that T inventory limit could be reached in ITER within only ~ 100 discharges and could therefore seriously impact the device operation unless efficient T removal processes are developed.
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